The invention pertains to the field of portable audio equipment.
1. Technical Field
With the widespread use of portable AM/FM receivers, portable AM/FM/Cassette players or cassette players standing alone and the rising popularity of portable CD players, a need has arisen for a good way of getting the audio signals to the user""s ears. Currently, users wear headphones which are coupled to the portable audio device by wires. These wires are inconvenient and possibly dangerous. Typically, users wear such portable audio devices while they are doing other things such as working out, rollerblading, doing manual labor, driving, cleaning the house etc. During such activities there is frequently a great deal of arm motion and sometimes there are moving or stationary objects which are or pass within close range of the users upper torso. The wires connecting headphones to the portable audio device usually swing freely in the space around the users upper torso since the typical place users place the portable audio device is along their waistline on a belt or in a special carrying case. As a result of the wires swinging freely in the space around the users upper torso between the users waist and the headphones, the wires frequently get tangled up with the users arms, or snag on some stationary or, worse, some moving object near the users upper torso. This can be not only destructive to the portable audio equipment and headphones but dangerous as well. Further, having the headphone yanked from your head and possibly sending eyeglasses flying with them can be embarrassing.
Therefore, a need has arisen for a device which can attach to a portable audio device and which can transmit audio signals from the portable audio device to the headphones without wires but which is small enough to be unobtrusive.
2. Background Art
Wireless FM transmitters from audio equipment to headphones with FM receivers therein are known. For example, Recotan makes a line of AC powered FM transmitters that receive audio signals from a stereo receiver, cassette player, CD player or DAT tape drive unit. These audio signals are modulated onto an RF carrier which is transmitted. The user wears headphones with receivers therein and can be anywhere within about 500 feet. The problem with this unit is that it must be plugged into a source of AC power and it is too large to use with a portable audio device. The Recotan units have external antennas which have disk-like shapes not unlike the radomes on E-3 AWACS airborne radar planes. Since there is no source of AC power on a portable audio device and since there is no room for such a large antenna structure, the Recotan units represent an unacceptable solution for a portable audio device.
Small RF transmitters have been used on electric guitars to transmit audio signals from the guitar transducer to a receiver coupled to a power amplifier. An example of this type of technology is found in U.S. Pat. No. 5,025,704 to Davis. In Davis, a small transmitter built into a plastic housing which has a phone plug projecting therefrom is plugged into a female phone jack on a guitar. The female phone plug is coupled to a transducer in the guitar which senses the vibrations of the strings and converts them to audio signals. The audio signals are modulated onto an RF carrier and the carrier is coupled to a coil of wire 50 which acts as an antenna. The coil of wire is wrapped around the circuit board inside the housing. The Davis patent also teaches that the metallic strings of the guitar are also coupled to signal ground of the transmitter and that the coil of wire 50 induces RF radiation between the coil of wire 50 and the strings of the guitar.
Other examples of RF transmitters for stringed instruments include U.S. Pat. No. 3,080,785 [Battery powered RF transmitter that modifies transducer acoustic signal from string vibrations to enhance aesthetic effect]; U.S. Pat. No. 3,085,460 [Battery powered tunnel diode RF transmitter coupled to transducer system of electric guitarxe2x80x94external rod antenna projecting from top of guitar near performer""s fret arm]; U.S. Pat. No. 3,743,751 [Drum sound effect unit small enough to fit inside electric guitar with signal from drum sound effect unit and transducer pickup for string vibrations summed and modulated onto RF carrier and transmitted via external antenna 90]; U.S. Pat. Nos. 3,825,666 and 3,901,118 [RF transmitter coupled to electric guitar]; U.S. Pat. No. 4,004,228 [portable transmitter with antenna conductor printed on printed circuit board with central area of printed circuit board with operative components carrying as much excess metal as possible so as to capacitively load the antenna and reduce the effect of any external conductors adjacent to the antenna and capacitively loading it and changing its impedance].
U.S. Pat. No. 4,344,184 teaches a wireless microphone not requiring external antenna. The microphone has a tubular housing is made of nonconductive material. A microphone projects from one end of the housing. The housing also contains an audio amplifier and a RF transmitter. The amplifier and transmitter are physically separated from each other, but are interconnected by RF chokes. The output of the transmitter is coupled to circuit ground of the audio amplifier such that the microphone and first circuit serve as one radiator of a dipole and the battery and second circuit serve as the other radiator of the dipole. U.S. Pat. No. 4,471,493 teaches a portable extension phone having an RF transmitter and an RF receiver therein, neither of which uses a whip antenna. The extension phone instead has a separate conductive plate which is part of a pivoting part of the phone which helps reflect sound from a user""s lips toward a built in microphone. The conductive plate also serves as one element of a dipole. The other element of the dipole is a conductive static shield which also serves to protect elements on a printed circuit board inside the extension phone.
U.S. Pat. No. 4,430,757 teaches a portable RF ELT transmitter which is contained within a housing which is hung around the user""s neck by a chain which forms a quarter wavelength antenna.
Other portable RF transmitters of interest include the transmitter taught in U.S. Pat. No. 2,840,694 to Morgan. This transmitter is basically a microphone that uses the microphone as the antenna. U.S. Pat. No. 4,794 622 teaches a low power, hand held RF transmitter which uses a surface acoustic wave resonator to stabilize the frequency of the RF oscillator to within 0.05% of the center frequency of the SAW resonator for the condition where the user""s hand is enclosing the transmitter. The stability of the RF transmitter frequency allows the receiver to use an extremely narrow bandwidth thereby cutting down on the amount of extraneous signals received by the receiver. U.S. Pat. No. 4,612,668 teaches a battery operated transmitter coupled to an audio input device by a cable. This transmitter uses a printed circuit loop conductor on the printed circuit of the transmitter both as a tank circuit inductance and capacitance which determines the frequency of operation as well as the antenna. Another conductive plate formed on the undersurface of the protective case that houses the transmitter so as to be in close proximity to the loop conductor inductor serves as the opposing plate of the tank circuit capacitor, the first plate of which is the loop conductor inductor. By setting the dimensions of this plate, the capacitance can be altered thereby changing the transmitter frequency.
A wireless headphone system is taught in U.S. Pat. No. 4,845,751. In this system, a headphone unit has both a transmitter and a receiver therein. A radio or other stereo unit has a transmitter/receiver plugged into an audio output jack. The audio output of the stereo unit is transmitted as two separate stereo channels to the headphone unit where the headphone receiver receives the signal and feeds each channel to one of the earpieces of the headset. The transmitter in the headphones encodes signals from seek and scan buttons on the headset and transmits these signals to the receiver of the transmitter/receiver unit plugged into the audio output jack. There, the signals are decoded and coupled to electronically controlled seek and scan controls for the stereo receiver. No indication is given in this patent as to whether the transmitter plugged into the stereo receiver is battery operated. The transmitters of the unit plugged into the audio output jack of the stereo receiver feed a wire antenna 38.
U.S. Pat. No. 2,828,413 teaches a portable radio receiver which is encapsulated in a conductive container which is divided into two halves which are separated by a dielectric. The two conductive halves of the container form a dipole antenna the halves of which are coupled to the two terminals of the primary winding of a transformer which has its secondary winding coupled to the RF input of the receiver.
According to principles of the present invention, there is provided a genus of portable, battery operated transmitters which are mounted in a case and which have no external antenna. The audio input to these transmitters is from some audio source such as a portable CD or cassette player or computer and is coupled to the transmitter through an earphone jack or audio output jack on the back of the computer. The antenna is a dipole antenna comprised of the ground conductive trace of the transmitter and the ground conductive trace of the audio source, the two ground conductors being connected at audio frequencies by a tri-filar RF choke, but isolated from each other at RF frequencies by the RF choke.
In some species, the transmitter is mounted in a casing which is integrally formed with an audio input plug jack which mates with an earphone or output jack on an audio source. As with all other species in the genus, the transmitter uses its own ground trace and that of the audio source as a short dipole antenna thereby eliminating the need for an external antenna.
In all species in the genus, the transmitter modulates the audio signal from the audio source onto an RF carrier and transmits it to a headphone receiver worn by a user who is also usually wearing or carrying the audio source.
One key aspect of all species in the genus of the present invention is the use of some means such as an RF choke to simultaneously electrically isolate the two ground traces at RF frequencies but to couple them by a low impedance path at audio frequencies, and to utilize the leakage inductance of the RF choke to neutralize or partially offset the capacitive reactance of the short dipole antenna. This transforms the impedance of the antenna structure into a principally real axis impedance for better power transfer. Another key aspect of at least some species in the genus of the present invention is the use of a transformer or tank circuit coupled to the RF output of the transmitter and the short dipole antenna to step down the impedance of the short dipole to an impedance that more closely matches the output impedance of the RF amplifier so as to increase power transfer to the antenna.
There are several alternative embodiments, some of which are described below. One embodiment uses an analog tuned RF transmitter with an internal oscillator in the transmitter chip tuned by an external tank circuit, and a headphone receiver which has two cascaded piezoelectric resonators in the IF section to increase the selectivity of the receiver and aid in rejecting interfering signals from adjacent frequencies to the transmitter frequency. Another preferred embodiment uses a PLL and microprocessor to digitally synthesize the carrier frequency at a frequency which is precisely located in the center of the channel the receiver is listening to eliminate distortion of the received signal that can be a problem in the analog embodiments if their frequency drifts off the center of the channel.